Understanding Fever

When is a high temperature not a fever?

What would you think was wrong with a horse that has been in a stall all night, refuses to eat his grain and has a rectal temperature of 103° F? Would you think the same thing was wrong with an equine athlete who registers the same temperature but has just come off of the cross-country course?

The answer is no! You should not assume that the same mechanisms apply to both of these horses with elevated temperatures. The horse just off the course has an elevated temperature because he has not been able to rid his body of the heat as rapidly as it built up. This can occur because of extreme environmental factors or because the horse does not produce adequate sweat. This is an overheated horse, not a horse with a true fever.

True fevers occur when the set point for the core body temperature is altered and the body gets messages to maintain its temperature at a new, higher setting. Triggers for altering the set point are called “pyrogens.” Pyrogens most commonly come into play when the horse encounters an infectious insult from bacteria or viruses.

Fever is considered to be a normal response to illness and has both beneficial and adverse effects for the patient. For example, fever can diminish the iron concentrations that many bacteria require for survival and replication. On the downside, prolonged fever can lead to decreased water and feed intake, muscle wasting and weakness. Foals can have fever-related seizures.

The most commonly used fever reduction drugs, Banamine (flunixin meglumine) and bute (phenylbutazone) will be effective only in the horse who has a true fever. Antibiotics will often be prescribed as well, but may not bring down a fever that is caused by an organism that is not sensitive to that particular antibiotic. We use rectal temperature to gauge response to treatment and often ask that the temperature be recorded two to four times per day so that we can alter medications logically.

It is wise to familiarize your horse with the use of the rectal thermometer so that if repeat readings become necessary, he or she will be cooperative! We will often ask for a temperature if you call us about a horse that is not acting right. The most common signs clients observe in a horse with a fever are lack of appetite, change in attitude, lethargy, wounds or swelling. Fortunately most of the patients we see for fever give us obvious reasons for their condition and usually respond positively to conventional treatment.